1. Field of the Invention
The present invention relates to an outboard motor which includes an engine and an exhaust pipe for leading an exhaust gas of the engine to a main exhaust passage arranged below the engine.
2. Description of Related Art
An outboard motor according to one prior art is disclosed in U.S. Patent Application Publication No. US 2008/0022669 A1. This outboard motor includes an engine which supports a crankshaft in a posture along the up-down direction, a casing positioned below the engine, and an engine cover surrounding the engine. The engine is a multi-cylinder engine. This outboard motor further includes an exhaust pipe which connects the engine and a main exhaust passage inside the casing to prevent exhaust interference among cylinders of the engine.
The engine is a V-shaped 6-cylinder engine having two cylinder rows which form a V shape in a plan view. Therefore, this engine includes two cylinder heads. In the side walls opposing each other of these two cylinder heads, exhaust outlets are formed, respectively.
The lower end of the crankshaft is connected to a drive shaft. The drive shaft is housed in the casing while being in a posture along the up-down direction. The lower end of the drive shaft is coupled to a propeller supported rotatably on the lower end of the casing.
Inside the casing, the main exhaust passage is formed so as to penetrate the inside of the casing in the up-down direction. The inlet end of the main exhaust passage is connected to exhaust outlets of the cylinder heads via an exhaust pipe. In addition, the outlet end of the main exhaust passage is opened inside water at the shaft center of the propeller.
The exhaust pipe consists of a tubular part forming exhaust passages of the respective cylinders and an exhaust chamber which collects the exhaust passages of the cylinders in one passage, and is housed in a rear part of the engine cover. The tubular part includes an exhaust manifold part positioned between the two cylinder heads. The tubular part includes a first horizontal part reaching the end on the right side of the outboard motor inside the engine cover from the exhaust manifold part. The tubular part further includes a second horizontal part reaching the end on the left side of the outboard motor through the rear end inside the engine cover from the tip end of the first horizontal part.
The second horizontal part is formed into an L shape in a plan view, and is arranged such that the corner at the bent portion is directed to the rear of the outboard motor. On the upstream side and the downstream side of the corner of the second horizontal part, catalysts are respectively provided.
The exhaust chamber is formed so as to extend in the up-down direction along the engine. To the upper end of the exhaust chamber, the tip end of the second horizontal part is connected. The inlet end of the main exhaust passage is connected to the lower end of the exhaust chamber.
The exhaust pipe of the outboard motor of the above-described prior art is formed so as to extend from the outboard motor right side to the outboard motor left side in a narrow space between the rear end of the engine and the rear inner wall surface of the engine cover. An exhaust gas flowing inside the exhaust pipe changes its direction from the engine to the outboard motor right side, and then changes its direction to the outboard motor left side. Therefore, the exhaust resistance is great, and there is a limit to improving the output of the outboard motor.